They further clarify the availability of illegally obtained statements for impeachment, and the application of the fruit of the poisonous tree doctrine.

Some states have provided greater protections for criminal suspects under their constitutions and statutes, but the emphasis here is on settled federal principles resolved by the United States Supreme Court (which set the baseline in state litigation as well).

The flowchart on the right (click on the image to get a full page PDF) will give you an overview of how these issues relate, but you need to launch the Q&A to get detailed explanations and links to the applicable Supreme Court cases.

Voluntariness. Statements are "involuntary" if they were the product of
coercion; that is, the suspect's will was overborne in the
totality of the circumstances. See
United States v. Dickerson, 530 U.S. 428 (2000).

Threats or promises (especially promises of immunity) may render a response involuntary, but this is not a per se rule; the test remains the totality of the circumstances. See
Arizona v. Fulminate, 499 U.S. 279 (1991). Similarly, tricks or lies used by police do not, by themselves, make a confession involuntary; they are simply factors to be weighed in the totality of the circumstances.
Frazier v. Cupp, 394 U.S. 731 (1969).

The admission of an involuntary confession violates the due process clauses of the Fifth and Fourteenth Amendments. State action (wrongdoing by a government actor) is an element of any due process claim. "Absent police conduct causally related to the confession, there is simply no basis for concluding that any state actor has deprived a criminal defendant of due process of law."
Colorado v. Connelly, 479 U.S. 157, 164 (1986) (police did not know that the suspect was insane).

Arguably, a state constitutional or evidentiary claim may be made without state action, but case law on that would be sparse and local.

Ordinarily, the actions of a private person do not trigger due process or self-incrimination concerns. See
Arizona v. Mauro, 481 U.S. 520 (1987). Consequently, as a matter of federal constitutional law, the statements obtained are admissible.

A fallback argument could be made in cases where there is strong evidence that the private party was acting as a government agent, but the Supreme Court has not spoken definitively. Evidentiary (involuntary statements are unreliable) and independent state constitutional grounds arguments may be available as well.

Statements that are involuntary may not be used in the government's
case-in-chief, or even for impeachment.
Mincey v. Arizona, 437 U.S. 385, 398 (1978).

A law enforcement agent's freedom of action is not significantly impacted here. Merely hiring an attorney does not trigger any protections under the Sixth Amendment, even if the suspect is the focus of a government investigation.
Hoffa v. United States, 385 U.S. 293 (1966).

Prosecutors may have ethical restraints that would prevent supervising such questioning. The parameters are set out in the version of ABA Model Code Disciplinary
Rule 7-104(A)(1).

The Sixth Amendment right to counsel is "offense specific." It does not apply to other crimes, even those that are "factually related" to the charged crimes. See
Texas v. Cobb, 532 U.S. 162 (2001) (indictment for burglary does not prevent questioning about murder of burglary victims).

In addition to attachment, a Sixth Amendment requires a critical stage. "Interrogation" after the Sixth Amendment right has attached is a critical stage.

The definition of interrogation under the Sixth Amendment is subtly different than the one used in the Fifth Amendment/Miranda context. Sixth Amendment interrogation occurs where police or cooperators (e.g., a cellmate or a wired accomplice) "deliberately and designedly set out to elicit information."
Brewer v. Williams, 430 U.S. 387, 399 (1977).

Custody (state or federal) may also trigger the requirement for Miranda
warnings. A suspect is in Miranda custody if "the relevant environment presents the same inherently coercive pressures as the type of station house questioning at issue in
Miranda."
Howes v. Fields, 565 U.S. 499, 509 (2012).

Section 3501(c) provides a six-hour "safe harbor." For interrogations outside the six-hour window, the government must show reasonable delay. Delay for the purpose of interrogation is unreasonable delay. See
Corley v. United States, 556 U.S. 303 (2009).

The Miranda rule comes into play where a suspect is subjected to custodial
interrogation by law enforcement. Volunteered statements are not the product of interrogation. "Interrogation" in this context is either express questioning or its "functional equivalent" (words that the questioner should know are reasonably likely to elicit an incriminating response).
Rhode Island v. Innis, 446 U.S. 291, 300-301 (1980).

Miranda rights are not required for routine booking in questions,
Pennsylvania v. Muniz, 496 U.S. 582 (1990), or where "a situation posing a threat to the public safety outweighs the need for the prophylactic rule."
New York v. Quarles, 467 U.S. 649 (1984).

If, during the colloquy, a suspect makes an ambiguous statement regarding the right to remain silent or right to counsel, it is good police practice to stop and clarify what the suspect means, but officers are not required to do so. Officers may proceed to interrogate the suspect without asking "clarifying questions."
Davis v. United States, 512 U.S. 452 (1994). Officers may not ask clarifying questions if the invocation is unambiguous, however.
Smith v. Illinois, 469 U.S. 91 (1984).

When there has been an unambiguous invocation of the right to remain silent, police must honor the invocation for a significant period of time. After that, however, they may approach the suspect with a fresh set of warnings, and attempt to obtain a waiver.
Michigan v. Mosely, 423 U.S. 96 (1976).

They may also make another attempt to obtain a waiver if the suspect, without prompting, initiates a further conversation with the police.
Oregon v. Bradshaw, 462 U.S. 1039 (1983). Also, they are free to approach the suspect if he or she is no longer in custody,
Bobby v. Dixon, 565 U.S. 23 (2011), or if there has been an intervening break in custody.
See
Maryland v. Shatzer, 559 U.S. 98 (2010).

An invocation of the right to silence does not rule out a subsequent waiver of
Miranda rights under these circumstances (honored invocation, initiation, break in custody), but the government must still show a waiver before there is further
interrogation.

Questioning must stop after an unambiguous invocation of the right to counsel.
Edwards v. Arizona, 451 U.S. 477 (1981). This bar on questioning remains effective indefinitely, even for questioning about unrelated crimes, and even if the second officer is unaware of the earlier invocation.
Arizona v. Roberson, 486 U.S. 675 (1988).

The bar may be lifted under certain circumstances, which differ depending on whether the police were seeking a Miranda or Sixth Amendment waiver when the suspect invoked.

Was the pre-invocation attempt to obtain a waiver necessary because of:

An Edwards invocation of the right to counsel does not prevent a later attempt to obtain a waiver where the suspect, without prompting, initiates a subsequent
conversation,
Oregon v. Bradshaw, 462 U.S. 1039 (1983), or where the suspect is no longer in custody when questioned again. See
Bobby v. Dixon, 565 U.S. 23 (2011).

Questioning is also permitted where the suspect was in custody at the time of the invocation, and is in custody again, but there has been an intervening 14-day break in custody.
Maryland v. Shatzer, 559 U.S. 98 (2010).

An Edwards invocation of the right to counsel does not prevent a later attempt to obtain a waiver where the suspect, without prompting, initiates a subsequent
conversation.
Oregon v. Bradshaw, 462 U.S. 1039 (1983).

Logically, the bar on questioning would also end after the charges have been dismissed, but the Supreme Court has not spoken on this.

The government must show a voluntary and intelligent waiver of Miranda rights.

Officers often conclude the warnings with a request for an express verbal or written waiver ("having these rights in mind, do you wish to go forward without a lawyer?"). However, an express waiver is not required. A waiver may be implied from the circumstances, even if the suspect refuses to sign the rights card. See
North Carolina v. Butler, 441 U.S. 369 (1979).

The voluntariness of a Miranda waiver is analyzed the same way as the voluntariness of a statement (the "totality of the circumstances" test). SeeButler,
id.

An intelligent waiver does not require a sophisticated understanding of things or the knowledge of all the relevant circumstances. It is intelligent if made "with a full awareness of both the nature of the right being abandoned and the consequences of the decision to abandon it."
Moran v. Burbine, 475 U.S. 412, 421 (1986).

The officer typically asks, right after reciting the warnings, whether the suspect understands the rights. Omitting that question makes proof of an intelligent waiver practically impossible.
Tague v. Louisiana, 444 U.S. 469 (1980). But see
Berghuis v. Thompkins, 560 U.S. 370 (2010) ("conflicting evidence"). But intelligent waivers will almost always be found where the suspect is asked the question, affirmatively indicates that he or she understands the rights, and goes on to answer questions.

The initial and later statements are involuntary.
Statements that are involuntary may not be used in the government's
case-in-chief, or even for impeachment.
Mincey v. Arizona, 437 U.S. 385, 398 (1978).

The initial statement is involuntary, and may not be used in the government's
case-in-chief, or even for impeachment.
Mincey v. Arizona, 437 U.S. 385, 398 (1978). However, the second untainted statement
was not so closely related that "the facts of one control the other." Consequently, it
is not fruit of the poisonous tree.

An involuntary confession does not necessarily taint a later confession. Where an otherwise admissible confession follows an involuntary confession, the test is whether the two statements are so closely related that "the facts of one control the other."
Leyra v. Denno, 347 U.S. 556, 561 (1954).

You have reached this point because the suspect did not receive proper Miranda warnings, and so there was not a valid waiver (e.g., police omit warnings, questioning by an informant supervised by police). This does not foreclose the admissibility of statements properly obtained at some later time.

This is true even if the initial statement was taken after required warnings were
intentionally omitted, and the officers knew that they were required, unless the unwarned statements were used in a "calculated way" to undermine later warnings. See
Missouri v. Seibert, 542 U.S. 600 (2004);
Bobby v. Dixon, 565 U.S. 23 (2011).

The questioning is unlawful, and statements obtained would be inadmissible in the government's case-in-chief. The statements may, however, be used to impeach the defendant if they were voluntarily made. See
Oregon v. Hass, 420 U.S. 714 (1975) (Miranda violations);
Kansas v. Ventris, 556 U.S. 586 (2009) (Sixth Amendment).

The initial questioning was unlawful, and statements obtained would be inadmissible in the government's case-in-chief. The statements may, however, be used to impeach the defendant if they were voluntarily made. See
Oregon v. Hass, 420 U.S. 714 (1975) (Miranda violations);
Kansas v. Ventris, 556 U.S. 586 (2009) (Sixth Amendment).

A defendant's statements in the second interrogation, however, if made voluntarily after a proper waiver of constitutional rights, are fully admissible.

The questioning is lawful under federal law. Special rules may apply under state constitutions, statutes, or evidence rules, but there do not appear to be any significant barriers to questioning under the U.S. Constitution or federal statutes.